The purpose of these investigations is to examine the host factors involved in the pathogenesis of life-threatening bronchiolitis or pneumonia of infants due to respiratory syncytial virus or parainfluenza virus type 3. Both of these viruses produce the greatest morbidity and mortality in infants less than 6 months of age when virtually all possess varying levels of homotypic maternal antibody. Hypotheses have been advanced which implicate immune mechanisms in the pathogenesis of these infections, and these mechanisms directly or indirectly involve passively-acquired maternal antibody. The proposed studies will examine in the hamster pneumonia model the relationship of varying levels of passive antibody to the parameters of infections with these viruses, and to the immune responses of the host. Previous studies of the model have demonstrated that passive antibody inhibits active serum antibody response. Important questions remain about the effect of passive antibody on the development of surface antibody, cell-mediated responses, interferon production and pulmonary macrophage function. Studies will be directed toward measuring the effect of passive immunization on these parameters. It is essential that the pathogenesis of these infections be defined before rational methods of prophylaxis can be considered.